Offshore structures are not unknown. In 1955 the U.S. Army Corps. of Engineers constructed radar stations along the New England coast, which were commonly referred to as “Texas Towers.” In constructing these radar stations, the radar platforms were lifted on supporting legs, using hydraulic cylinders. While the legs and the platform were pinned together, a plurality of hydraulic cylinders were manually attached between the supporting legs and the platform. The pins holding the platform stationary with respect to the legs were removed, and the hydraulic cylinders were then pressurized to extend their pistons and raise the radar platform. At the end of the pistons' strokes, the pins holding the platform in position with respect to the supporting legs were manually replaced to hold the platform in a stationary position with respect to the legs so the plurality of cylinders could be disconnected from the platform and the legs, and their pistons could be retracted without affecting the relative positions of the platform and the legs. The plurality of hydraulic cylinders were then manually reattached between the platform and the legs, and the pins holding the platform stationary with respect to the legs were manually removed, and the hydraulic cylinders were operated again to extend their pistons and raise the platform with respect to the legs. This procedure was repeated again and again until the platform was lifted to its desired position with respect to the plurality of legs. This method of construction was labor-intensive, slow, and expensive.
The increasing need for oil and gas has led to offshore exploration, requiring drilling into the earth's surface far below the water. Such drilling operations are accomplished from mobile offshore drilling units (MODUs). MODUs generally comprise submersible, semi-submersible and jack-up types, with which the invention is concerned. Jack-up MODUs are massive structures which can have platform surface areas as large as two acres to support the drilling equipment, drilling supplies, power sources, living quarters, helicopter landing ports, and the stores and fuel that are necessary to maintain a drilling crew and operate the MODU and its drilling equipment hundreds of feet above the underwater surface. Jack-up MODUs include a plurality of MODU supporting legs, most generally three legs, that are moveably engaged with the MODU platform. Following their construction, such MODUs, with their MODU platforms resting on footings at the base of each supporting leg are towed to an offshore drilling site, like a large vessel with three 700 foot masts. Once the MODU is positioned at a drilling site offshore, the MODU supporting legs are lowered to engage the earth's underwater surface and thereafter lift, or jack-up, the MODU platform sufficiently above the water level to reduce exposure of the MODU platform to wave action during severe storms. It is not uncommon for jack-up MODUs to weigh 30,000 to 40,000 tons, or more, with the MODU platform and its variable loads comprising as much as two-thirds of the weight. In addition, it is not uncommon for the MODU supporting legs to have lengths of 600 to 700 feet, and, to provide stability in their support of the MODU platform, to have cross sections, most commonly triangular, up to 50 feet on a side.
The jack-up MODUs currently in use and being constructed include, as the apparatus to adjust the relative position of the MODU platform and MODU supporting legs, a plurality of motor-driven spur gears which engage toothed racks running the length of each corner leg chord of each MODU supporting leg. The leg chords that comprise the corners of the MODU supporting legs of such currently existing jack-up MODUs are constructed with a central toothed rack, of expensive high strength (e.g, 100 KSI) steel, running the length of the supporting leg, with rigidifying semi-circular, tubular structural members welded along both sides of the toothed rack to increase the strength, section modulus and rigidity of the leg chords. Because the spur gears rotationally engage the toothed racks of the leg chords in raising and lowering the MODU supporting legs with respect to the MODU platform, the spur gear teeth and the teeth of the leg chord racks have cycloidal cross sections, and the spur gear drives are each engaged with the leg chord racks by line contact between a single tooth of the spur gear and a single mating tooth of a toothed rack, exposing the teeth of both the spur gear and the rack to extremely high shear forces and requiring that the spur gears and the toothed rack be made of an expensive high-grade steel, with a modulus of elasticity, for example, of 100,000 pounds per square inch (100 KSI).
Because of the great weights being handled and the high stress engagement between the spur gear teeth and rack teeth, as many as 18 spur gear drive units may be engaged with the six toothed racks on each supporting leg. In such systems, the plural spur gear drives are mounted vertically in sets of three units, one above another, so their pinion gears can engage the toothed racks that comprise the leg chords; however, the load is unequally shared by the plurality of engaged pinion gears, the lowest pinion gear and its engaged rack tooth carrying a significantly disproportionate portion of the load. Because the tooth loading in current spur gear driven jack-up MODUs is approaching the stress and fatigue limits of the available materials, complex controls for the electric motors of the spur gear drives have been developed in an effort to equalize the loads that are borne by the plurality of engaged gears and the associated stresses and fatigue. Such controls control the torques generated by the electric motors to balance the loads on their pinion gears and gradually accelerate and decelerate in an effort to avoid overstressing and fatiguing the engaged teeth. Further, during operation of the spur gear drives, grease must be mopped onto the rack teeth by the MODU crew to reduce the friction between the pinion gears and the leg chord racks, and the grease inevitably falls into the sea.
In addition to requiring expensive controls, materials and manufacturing procedures, spur gear-driven jack-up MODUs also require expensive separate locking apparatus for each supporting leg to maintain the MODU platform in a stationary position with respect to its supporting legs
The jacking systems of jack-up MODUs are currently expensive to design and manufacture and are not expected to satisfy future requirements. There is an increasing demand for larger jack-up MODUs with dramatically greater topside loads. The ability to meet this demand has, however, approached its practical limit with existing materials and technology, and a new jack-up MODU and MODU jacking system are needed.